1. Field of the Invention
The present invention relates to a method and an apparatus for feedback control of the air-fuel ratio of an air-fuel mixture at a desired value by means of an air-fuel ratio sensor positioned in the exhaust gas pipe in automobiles or the like.
2. Description of the Prior Art
A known feedback (closed-loop) control method for controlling the air-fuel ratio repeats the following steps so as to control the center value of the controlled air-fuel ratio within a very narrow range of air-fuel ratios around the stoichiometric ratio required for reducing and oxidizing catalysts. First, the running speed of the engine and the intake-air amount are detected. Then a basic fuel injection quantity supplied to fuel injection valves is calculated in accordance with the detected engine speed and the intake-air amount. The basic fuel injection quantity is corrected by using an air-fuel compensation factor (normal correction factor) which is calculated from detection signals indicative of the cooling water temperature, the intake-air temperature, and the like. Thus, the corrected fuel injection quantity determines the actual fuel-feeding rate of the engine.
The above-mentioned narrowly controlled center value of the air-fuel ratio is affected by the characteristics of the air-fuel ratio sensor, the exhaust gas composition characteristics, and the like. That is, the controlled center value of the air-fuel ratio often deviates from an optinum value as a result of the individual differences in the control characteristics of the parts of the engine due to aging of the engine or due to environmental changes.
In order to compensate for the individual differences in the parts of the engine, another air-fuel compensation factor which is called a learning correction factor is introduced to maintain an optinum air-fuel ratio. In this case, the basic fuel injection quantity is corrected by using two kinds of air-fuel compensation factors.
The learning correction factors (second air-fuel compensation factors) are also determined by the operating conditions of the engine, such as the engine speed and the intake-air quantity. In addition, the learning correction factors themselves are corrected by a detection signal from the air-fuel ratio sensor.
In the prior art, however, such correction of the learning correction factors is performed at every predetermined crank angle of the engine so that variance of the learning correction factors becomes large due to variance of the engine speed, with the result that the air-fuel ratio is not accurately controlled. In addition, even when the engine is in a transient operating condition, such as an accelerating or decelerating condition, correction of the learning correction factors is performed so that the air-fuel ratio after being controlled often deviates from an optimum value. As a result, when the feedback loop is opened, that is, when the feedback operation is stopped, the stoichiometric air-fuel ratio cannot be controlled so as to deteriorate the emission characteristics of the engine, the malfunctional initiation of the engine, and the like.
Note that the above-mentioned basic fuel injection quantity and two kinds of air-fuel compensation factors, that is, normal correction factors, integration (proportion) correction factors, and learning correction factors, are usually stored in a memory.